tipc: fix an information leak in tipc_topsrv_kern_subscr

tipc: fix an information leak in tipc_topsrv_kern_subscr

Use a 8-byte write to initialize sub.usr_handle in
tipc_topsrv_kern_subscr(), otherwise four bytes remain uninitialized
when issuing setsockopt(..., SOL_TIPC, ...).
This resulted in an infoleak reported by KMSAN when the packet was
received:

  =====================================================
  BUG: KMSAN: kernel-infoleak in copyout+0xbc/0x100 lib/iov_iter.c:169
   instrument_copy_to_user ./include/linux/instrumented.h:121
   copyout+0xbc/0x100 lib/iov_iter.c:169
   _copy_to_iter+0x5c0/0x20a0 lib/iov_iter.c:527
   copy_to_iter ./include/linux/uio.h:176
   simple_copy_to_iter+0x64/0xa0 net/core/datagram.c:513
   __skb_datagram_iter+0x123/0xdc0 net/core/datagram.c:419
   skb_copy_datagram_iter+0x58/0x200 net/core/datagram.c:527
   skb_copy_datagram_msg ./include/linux/skbuff.h:3903
   packet_recvmsg+0x521/0x1e70 net/packet/af_packet.c:3469
   ____sys_recvmsg+0x2c4/0x810 net/socket.c:?
   ___sys_recvmsg+0x217/0x840 net/socket.c:2743
   __sys_recvmsg net/socket.c:2773
   __do_sys_recvmsg net/socket.c:2783
   __se_sys_recvmsg net/socket.c:2780
   __x64_sys_recvmsg+0x364/0x540 net/socket.c:2780
   do_syscall_x64 arch/x86/entry/common.c:50
   do_syscall_64+0x3d/0xb0 arch/x86/entry/common.c:80
   entry_SYSCALL_64_after_hwframe+0x63/0xcd arch/x86/entry/entry_64.S:120

  ...

  Uninit was stored to memory at:
   tipc_sub_subscribe+0x42d/0xb50 net/tipc/subscr.c:156
   tipc_conn_rcv_sub+0x246/0x620 net/tipc/topsrv.c:375
   tipc_topsrv_kern_subscr+0x2e8/0x400 net/tipc/topsrv.c:579
   tipc_group_create+0x4e7/0x7d0 net/tipc/group.c:190
   tipc_sk_join+0x2a8/0x770 net/tipc/socket.c:3084
   tipc_setsockopt+0xae5/0xe40 net/tipc/socket.c:3201
   __sys_setsockopt+0x87f/0xdc0 net/socket.c:2252
   __do_sys_setsockopt net/socket.c:2263
   __se_sys_setsockopt net/socket.c:2260
   __x64_sys_setsockopt+0xe0/0x160 net/socket.c:2260
   do_syscall_x64 arch/x86/entry/common.c:50
   do_syscall_64+0x3d/0xb0 arch/x86/entry/common.c:80
   entry_SYSCALL_64_after_hwframe+0x63/0xcd arch/x86/entry/entry_64.S:120

  Local variable sub created at:
   tipc_topsrv_kern_subscr+0x57/0x400 net/tipc/topsrv.c:562
   tipc_group_create+0x4e7/0x7d0 net/tipc/group.c:190

  Bytes 84-87 of 88 are uninitialized
  Memory access of size 88 starts at ffff88801ed57cd0
  Data copied to user address 0000000020000400
  ...
  =====================================================

Signed-off-by: Alexander Potapenko <glider@google.com>
Fixes: de38a8cdce81fb ("tipc: rename tipc_server to tipc_topsrv")
Signed-off-by: David S. Miller <davem@davemloft.net>